The present invention provides methods and apparatus for repairing inoperative pixels in a display. In particular, the present invention provides methods and apparatus for improving the effective yield rates of displays, such as liquid crystal micro-displays, by disconnecting inoperative pixels from their defective drive circuitry and tying such pixels to the working drive circuit of a nearby pixel.
Manufacturing of displays, such as liquid crystal (LC) micro-displays, with very large numbers of pixels is hampered by low yield rates due to the large area of semiconductor material (e.g., Complementary Metal Oxide Semiconductor (CMOS)) required for each device. Nevertheless, displays can actually tolerate a certain level of various types of defects and still be considered acceptable. Additionally, there are various techniques that have been developed and successfully applied to other display technologies that can reduce the visibility of some types of defects and dramatically increase the yield of acceptable displays.
For example, on a device size suitable to hold 8 million pixels (approximately 32×58 mm), without any repair strategy, the yield of “perfect” display devices is estimated at less than 2% (assumed defect rate of 0.2/cm2).
Defects in the microelectronic circuitry can cause a variety of types of malfunctions in the resulting display, such as:
Stuck-ON PixelThese pixels are always ON. In athree-chip RGB (red, green, blue)system, they would be visible as aconstant red, green, or blue dotin the display. Such stuck-ONpixels are much more visible indark areas of the display thanbright areas.Stuck-OFF PixelThese pixels are always OFF. In athree-chip RGB system, they wouldbe visible as a constant cyan,magenta, or yellow dot in thedisplay. Such stuck-OFF pixels aremuch more visible in the brightareas of the display than the darkareas.Stuck-IntermediateSome pixels can get stuck in anintermediate state. Depending onthe intended color of the pixeland the surrounding area of thedisplay, they will sometimesappear as red, green, or blue, andat other times will appear as thecomplement of the color channelaffected.Partial ResponseSome pixels may have a portion oftheir circuitry affected such that theypartially respond to the intendedcolor. Perhaps they are of reducedintensity or contrast, but theystill track the intended value to somedegree.Defective ClumpA significant portion of thecircuitry in the visible area maybe shared between clumps ofpixels. A defect in this area mayeffect the entire clump of pixelsand may take any of the aboveforms.Dead Column/RowDefects in signals generated bythe drive circuitry which are fedto every pixel in a column or rowwill manifest themselves in theentire column or row. Themanifestation may take a varietyof forms.
Although a display having any of the foregoing defects may still be useable, all such defects are significant enough that once noticed, the user will be unhappy. The only exception being perhaps a few isolated stuck-OFF pixels in the display used for the blue channel, due to the low contrast sensitivity of human vision to blue light.
A previously known method for dealing with such defects is to provide redundant circuitry in the display that can be selected to drive a pixel or column of pixels in the event of a defect in the primary circuitry. As it is already difficult to fit all required circuitry in the space available in such micro-displays, the inclusion of redundant circuitry becomes problematic.
It would be advantageous to provide a method of repairing inoperative pixels in a display without requiring redundant circuitry in order to increase the yield rate of such displays.
The methods and apparatus of the present invention provide the aforementioned and other advantages.